Common mode control circuitry for multi-stage operational amplifiers

ABSTRACT

This disclosure relates to a common mode regulation in multi stage differential amplifiers.

BACKGROUND

In general, in electronic circuits implementing a fully differentialamplifier, a need exists for the fully differential amplifier to controlcommon-mode voltage at its output. A typical approach includes using acommon mode feedback (CMFB) amplifier. A CMFB amplifier can providecontrol of the common mode voltage at different nodes that cannot bestabilized by negative differential feedback, where a reference voltagemay be provided that can provide a maximum differential voltage gainand/or maximum output voltage swing. The CMFB amplifier can also providesuppression of common mode components that tend to saturate differentstages, by applying common mode negative feedback.

A CMFB amplifier senses the common-mode voltage or V_(A) at the output;compares V_(A) with a reference voltage or V_(ref), and uses a controlvoltage V_(cntrl) to control voltage of an internal biasing node of theCMFB amplifier, and provides feedback regulation. In a typical Milleroperational amplifier (Op Amp), the internal biasing node is in theactive load of a first stage.

The CMFB amplifier circuit may be realized with continuous-time orswitched-capacitor structures. Such structures should have a speedperformance comparable to the unity-gain frequency of the differentialpath, otherwise noise from power supplies could be significantlyamplified (i.e., power supply rejection would be too small). In aconventional CMFB amplifier, this is usually difficult, since the CMFBamplifier may rely for stability on the differential path compensation.For example, the CMFB path in a Miller amplifier has an additional polewith respect to the differential one which is located at the controlnode, and the CMFB bandwidth will necessarily be smaller than thedifferential bandwidth. Depending on the circuit, the bandwidth may beup to two or three times smaller.

Additionally, in structures that do not rely only on Millercompensation, such as multistage amplifiers without Miller compensation,the conventional CMFB amplifier cannot be fully stabilized using thecompensation of the differential path.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Thesame numbers are used throughout the drawings to reference like featuresand components.

FIG. 1 is a block diagram illustrating an exemplary device forimplementing fully differential amplifiers that provided a stable commonmode (CM) regulation.

FIG. 2 is a block diagram illustrating an exemplary three-stage fullydifferential amplifier using common mode regulation.

FIG. 3 is a circuit diagram of an exemplary three-stage fullydifferential amplifier using the invented common mode regulation.

FIG. 4 is a flowchart illustrating an exemplary method for common moderegulation in multistage fully differential amplifiers.

DETAILED DESCRIPTION Overview

The disclosure is directed to techniques for controlling common mode(CM) voltage of fully differential amplifiers. The techniques andconcepts that are disclosed can be applied to fully differentialamplifiers to provide a stable CM regulation. In particular, thedisclosed techniques may be applied when relatively high bandwidth CMregulation is desired, since there is no dependence on the fullydifferential amplifier bandwidth. The techniques can be used togetherwith different kinds of compensation techniques, and are not limitedwith use only with Miller compensated amplifiers. Different compensationtechniques include techniques where the classic common mode feedback(CMFB) amplifier circuit cannot be used, for example, multistageamplifiers and conditionally stable amplifiers.

Exemplary System

FIG. 1 illustrates an exemplary device 100 for implementing fullydifferential amplifiers that provided a stable common mode (CM)regulation. System or device 100 may be one of various electronicdevices, such as a communication device (e.g., cellular telephone).Device 100 may include other components that are not shown in FIG. 1,such as processors), memory(ies), and various interfaces (e.g., intrasystem busses, antennae, etc.)

In the example, device 100 includes a digital front end 102 thatcommunicates with an analog front end 104. The analog front end 104 maybe a digital to analog converter (DAC) or include a DAC, when referringto a transmit path. The analog front end 104 can include connectivefilters or filters 106. The filters 106 may further include operationalamplifier(s) or Op Amps 108, which are fully differential Op Amps, andmay apply to a receive path. The Op Amps 108 are further discussedbelow. Device 100 is an exemplary implementation of Op Amps 108. It iscontemplated that other implementations can make use of the techniquesand methods describing Op Amps 108 and the concepts discussed below.

Exemplary Amplifier

FIG. 2 illustrates an exemplary differential amplifier or amplifier 200.Amplifier 200 may be an implementation of Op Amps 108 described above.The amplifier 200 implements common mode voltage regulation that usesfeed back and feed forward regulation paths. As further discussed below,the feed back path may function like a traditional CMFB amplifier, withits bandwidth related to the differential path compensation. If Millercompensation is implemented for the amplifier 200, or for one of thestages of amplifier 200, this part of the circuit of amplifier 200,provides the common-mode regulation for the low-mid frequency range,with a bandwidth limited by the differential path compensation.

As further discussed below, the common mode feed forward path senses anoutput common-mode voltage and compares it with a reference voltage inorder to inject a common-mode current in an amplification stage ofamplifier 200. This provides common mode regulation. Having a zero inits transfer function, the common mode feed forward path provides commonmode voltage regulation in the high frequency range.

The combination of the two feed back and feed forward paths can providea very stable and wide band regulation, by placing the zero of the feedforward path in such a way that it compensates the non dominant pole ofthe feed back path. Furthermore, such an implementation provides thatbandwidth of the common mode voltage regulation is not fully linked tothe differential path bandwidth, and the bandwidth of the common modevoltage regulation can be enlarged, by increasing the power of the feedforward path.

Amplifier 200 may be implemented as a relatively high speed Op Amp, andparticularly Op Amp(s) that do not use simple Miller compensation, suchas three (or more) stage amplifiers. In general, an amplifier or stagemay be needed for each of the feed back and feed forward paths of thecommon mode regulation; however, provided that the two paths do not acton the same stage, a single amplifier can be used.

Amplifier 200 is described as a three stage amplifier; however, it is tobe appreciated that the concepts discussed may apply to amplifiersimplementing various number of stages. Amplifier 200 has a firstamplification stage that includes two amplifiers or stages A′ 202 and A204. Stage A′ 202 and stage A 204 receive differential inputs IN_(N) 206and IN_(p) 208. A second amplification stage, stage B 210, is coupled tothe first amplification stage through stage A 204. A third amplificationstage, stage C 212, is coupled to stage B 210.

Amplifier 200 is designed for high gain and uses a single Millercapacitor CM 214(1) and CM 214(2) to compensate the non dominant polebetween the first stage A 204, and the third stage, stage C 212. Asingle Miller capacitor is used in reference to Miller compensation usedto compensate one pole. The non dominant pole introduced by the secondstage, stage B 210, is compensated with a zero introduced with a feedforward signal path by stage A′ 202, as further discussed below.

When such a design is used for a fully differential structure, aconventional CMFB amplifier may not be used, since the single Millercapacitor CM 214 may not be enough to compensate the common-mode loop.While the CMFB amplifier shares poles with the differential path, havingeven one more at a control node, the CMFB amplifier is not influenced bythe zero added with the feed forward signal path, and the common modeloop may only be partially compensated. Therefore, a feed forward pathfor the common mode helps to stabilize the common-mode loop, increasingits bandwidth to a value comparable to the differential path.

As discussed, stage A′ 202 and stage A 204, which make up a first stage,stage B 210 that makes up a second stage, stage C 212 that makes up athird stage, are amplification stages of amplifier 200. A feed forwarddifferential path is provided by stage A′ 202 injecting a currentsignal(s) into the output of the second stage, stage B 210.

A non dominant pole introduced by C 212 is compensated with the Millercapacitor CM 214 introduced between the output of stage A 204 and theoutput of stage C 212. The non-dominant pole introduced by B 210 iscompensated by the zero introduced by Stage A′ 202.

Common mode control loop is described as follows. The amplifier 200provides a differential voltage output shown as OUT_(N) 216 and OUT_(P)218, and defined by common mode voltage V_(CM-OUT) 220. The value ofV_(CM-OUT) 220 may be sensed, by summing resistive network or similarcomponent, as represented by sense 222. The value of V_(CM-OUT) 220 iscompared with a reference voltage V_(CM) 224 at an amplifier D 226.

One of the outputs of amplifier D 226, is used to feed back a controlvoltage V_(CTRL) 228 into Stage A 204. This is the part of the amplifier200, which acts as a conventional CMFB amplifier; the path isrepresented by line 230. In a differential stage, V_(CTRL) 228 controlsthe biasing of the active load.

The other output of amplifier D is used to control two current sources232(1) and 232(2), which inject a feed forward common-mode current(s)I_(CTRL) 234(1) and I_(CTRL) 234(2) into stage B 210; the path isrepresented by line 236. In practice, the two currents I_(CTRL) 232(1)and I_(CTRL) 232(2), which are the same current since controlled from asingle node, change the biasing of the amplifying devices of the secondstage, Stage B 210.

Combining the two paths represented by lines 230 and 236, so that thezero introduced by the feed-forward also partially compensates the nondominant pole of stage B 210, resulting in a fully stable completecommon-mode loop transfer function. Furthermore, it is possible toincrease the bandwidth of the common-mode regulation loop, by increasingpower in the feed-forward path (as represented by line 236).

FIG. 3 illustrates an exemplary circuit 300 of a fully differentialthree stage amplifier. As an example, the circuit 300 may be implementedusing 65 nm CMOS technology with a one volt power supply. Circuit layout300 is an exemplary implementation of amplifier 200. It is to beappreciated, that other implementations or configurations are possible,using different discrete components and/or arrangement of suchcomponents. Corresponding stages (amplifiers) and components areparticularly identified by similar numerals identifying components(elements) in FIG. 3, as shown and discussed in reference to FIG. 2.

The first stage of the differential amplifier circuit 300, correspondingto stage A 204, is a folded cascode with a pMOS input differential pair,a cascode pair A 302(1) (as identified by the dotted line) and an activeload A 302(2) (as identified by the dotted line). The stage A 204provides a high gain, helping to create a dominant-pole system.

The second stage is done with a pMOS in common source configurationcorresponding to stage B 210. A simple current mirror(s) B 304(1) and B304(2) provides necessary phase inversion. Moreover, a compensationresistor(s) R 306(1) and R 306(2) is connected to the gate of thereference side of this current mirror(s) B 304 to enhance speed andbandwidth. The feed-forward differential path is a pMOS differentialpair which provides the high frequency signal path from the OpAmp inputsIN_(P) 206 and IN_(N) 208 to the output of the second stage (B 210),corresponding to A′ 202 in FIG. 2. Therefore, no compensationcapacitance is needed for the second stage.

CM 214 is the Miller capacitor, connected between the output of A 204and the output of the third stage (stage C). The third stage C 212, is anMOS in common source configuration. Biasing voltages are identified byVb1 308(1), Vb2 308(2), Vb3 308(3), Vb4 308(4), and Vb5 308(5).

Common mode (CM) regulation block 310 includes summing resistive networkor sense 222 and amplifier D 226. For CM regulation, common-mode voltageis sensed between OUT_(N) 216 and OUT_(P) 218 and compared with thereference V_(CM) 224 at amplifier D 226. The feed back path injects thecontrol signal V_(CTRL) 228 into the folded cascode of the first stage,and provides a negative feed back. The feed forward path which providesa pole-zero cancellation, injects a common-mode current 234(1) and234(2) into the second stage B 210, regulating the common-mode voltageby acting on the biasing of this amplification stage.

Exemplary Methods

The order in which the methods below are described is not intended to beconstrued as a limitation, and any number of the described method blockscan be combined in any order to implement the methods, or an alternatemethod. Additionally, individual blocks may be deleted from the methodswithout departing from the spirit and scope of the subject matterdescribed herein.

The methods introduced may, but need not, be implemented at leastpartially in architecture(s) such as shown in FIGS. 1 to 3. In addition,it is to be appreciated that certain acts in the methods need not beperformed in the order described, may be modified, and/or may be omittedentirely. Furthermore, the methods can be implemented in any suitablehardware, firmware, or a combination thereof, without departing from thescope of the invention.

FIG. 4 is a flowchart 400 illustrating an exemplary method for commonmode regulation in multistage fully differential amplifiers.

At block 402, an output voltage is sensed at a multistage fullydifferential amplifier. The sensed output voltage, VCM OUT, is sensed atan output final stage of the multistage differential amplifier. Thesensed output voltage, VCM OUT, is representative of the output commonmode voltage of the multistage amplifier. In the discussion above, VCMOUT is described as V_(CM-OUT) 220. As discussed above, this sensing maybe performed by one of various circuits, such as a summing resistivenetwork.

At block 404, comparison is made as to the sensed output voltage, VCMOUT, and a common mode reference voltage, VCM REF. The comparison may bemade by an amplifier, such as amplifier D 226 as discussed above.

At block 406, the difference between VCM OUT and VCM REF, as performedin block 404 is amplified. This amplified difference provides a loopgain for the common mode signal.

At block 408, a feed forward current is injected to a stage of themultistage amplifier, and in parallel a feed back control voltage isprovided to another stage. As discussed above the feed forward currentis used to change biasing of an intermediate amplifier or amplificationstage. The feed back control voltage changes the biasing of an activeload of an initial or first stage, performing as a conventional commonmode feedback (CMFB) amplifier.

CONCLUSION

For the purposes of this disclosure and the claims that follow, theterms “coupled” has been used to describe how various elementsinterface. Such described interfacing of various elements may be eitherdirect or indirect. Although the subject matter has been described inlanguage specific to structural features and/or methodological acts, itis to be understood that the subject matter defined in the appendedclaims is not necessarily limited to the specific features or actsdescribed. Rather, the specific features and acts are disclosed aspreferred forms of implementing the claims. For example, the systemsdescribed could be configured as monitoring circuits and incorporatedinto various feedback and control loops. In addition, the voltageregulation circuit may include other types of devices and amplifiers invarious analog and digital configurations.

1. A fully differential amplifier comprising: a first amplificationstage; a second amplification stage coupled to the first amplificationstage, wherein a feed forward path is provided by injecting a currentsignal to the output of the second amplification stage; a thirdamplification stage coupled to the second amplification stage, having anon dominant pole that is compensated by a single Miller capacitor; anda common mode amplifier that provides an output to control a currentsource that injects a control current into the second amplificationstage.
 2. The fully differential amplifier of claim 1, wherein the firstamplification stage is comprised of two stages.
 3. The fullydifferential amplifier of claim 1, wherein the first amplification stageprovides a high gain that creates a dominant pole system.
 4. The fullydifferential amplifier of claim 1, wherein the first amplification stageincludes a folded cascade.
 5. The fully differential amplifier of claim1, wherein the third amplification stage provides a differential outputof the fully differential amplifier.
 6. The fully differential amplifierof claim 1, wherein a voltage of a differential output is sensed andcompared with a reference voltage, the difference of the voltage of thedifferential output and the reference voltage is amplified to provide aloop gain for a common mode signal.
 7. A method for common moderegulation in a fully differential amplifier comprising: sensing anoutput voltage of the fully differential amplifier; comparing the outputvoltage to a reference voltage; determining the difference of the outputvoltage and the reference voltage; amplifying the difference to provideloop gain for a common mode signal; injecting a feed forward currentbased on the loop gain, to one stage of the fully differentialamplifier; and providing a feed back control voltage to another stage,in parallel to the injecting, the feed back control voltage to change abiasing of an active load of the another stage.
 8. The method of claim7, wherein the output voltage is representative of a common mode outputvoltage of the fully differential amplifier.
 9. The method of claim 7,wherein the sensing is performed by a summing resistive network.
 10. Themethod of claim 7, wherein the determining and amplifying is performedby a common mode amplifier.
 11. The method of claim 7, wherein the feedforward current changes biasing of the second stage.